Can a 6.5l engine be re-sleeved? and if so is there a kit with the piston and rings for one cylinder?

You can buy the rings and such, but the 6.5 is a parent bore engine

Yes it can be sleeved and you can buy a single piston as well.

I also have another question. I had a valve collapse. the spring was broke and caused the valve to break off. the rocker arm holder was also broken. I am boring the cylinder and purchased a new head. I am wondering what could have been the cause apart from the spring just being weak and breaking.

bad spring + bad luck